Fluid compressor



Sept. 27, 1966 F. o. E. SCHULTZ 3,

FLUID COMPRESSOR Filed April 6, 1964 2 Sheets-Sheet 1 2 I INVENTOR.

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p 27, 1966 F. o. E. SCHULTZ 3,275,225

FLUID COMPRESSOR Filed April 6, 1964 2 Sheets-Sheet 2 OUTLINE OF AcTw/L07012.

OUTLINE OF INVENTOR. .FUHHEST [7.5. 501% T2.

United States Patent 3,275,225 FLUID COMPRESSOR Forrest O. E. Schultz,Owosso, Mich., assignor to Midland-Ross Corporation, Cleveland, Ohio, acorporation of Ohio Filed Apr. 6, 1964, Ser. No. 357,561 8 Claims. (Cl.230-141) This invention relates to improved apparatus for compressingfluids and particularly compressible fluids such as air. Moreparticularly, the invention relates to an improved compressor of thelobed-rotor type, frequently called a Roots blower or compressor afteran early patentee of the configuration (Reissue Patent 2,369 of October2, 1866).

A typical lobed-rotor compressor comprises two rotating elements orrotors which are caused to rotate in opposite directions about spacedparallel axes in an appropriately contoured chamber. Each rotor isprovided with a number of spaced apart outwardly extending lobes,

usually two, and an equal number of recesses alternately spaced betweenthe lobes. In current practice the alternate lobes and recesses usuallycomprise arcs of circles of equal diameter.

In such an arrangement, the rotation of the rotors is so synchronized,as by timing gears, and the spacing between the rotor axes is somaintained, that some predetermined clearance is continually maintainedbetween the rotors as the lobe of one rotor and the recess of the otherrotate into and out of register with one another. The purpose of thisclearance is, of course, to prevent wear which would arise if the rotorsWere permitted to rub against one another. However, the clearance soprovided between the rotors inherently constitutes a path for compressedfluid to escape back from the high pressure side of the compressor tothe low pressure side of the compressor with consequent loss ofvolumetric efficiency of the compressor. Defined as slip, this leakageis a constant for any given compressor at a given pressure and isindependent of compressor speed. Hence, compressor volumetric efficiencyfor a lobed-rotor compressor decreases substantially as the compressorspeed is reduced from the maximum or rated speed. This characteristic oflobed-rotor compressors has generally limited their commercial utilityto substantially constant speed (constant output volume) applications.

In accordance with the present invention, however, there is provided alobed-rotor compressor having rotors of a novel configuration which isadvantageous in minimizing compressor slip and which imparts commercialutility to the compressor for applications requiring Wide variation incompressor speed. A particular application for such a compressor is inproviding compressed air for the ManAirOx (manifold air oxidation)automotive exhaust purification system which is being developed pursuantto the anti-smog campaigns of Los Angeles and other communities. Typicalrequirements for a compressor for such an application are that it beable to provide 230 cubic feet per minute (standard conditions) of airat 0-10 p.s.i.g. pressure over an operating speed range of 1000-10,000revolutions per minute. Prior to the present invention no I knownlobed-rotor compressors were able to satisfactorily meet theserequirements.

For a further understanding of the invention, attention is directedtothe following portion of the specification, the drawing, and theappended claims.

In the drawing:

FIG. 1 is an elevational sectional view of a compressor embodying thepresent invention;

FIG. 2 is a sectional view taken on line 22 of FIG. 1; FIG. 3 is anoutline view of a rotor showing the geometrical configuration of itsperiphery.

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FIGS. 4 and 5 are outline views showing the clearance between rotors,and between each rotor and the wall of the surrounding chamber, forvarious positions of rotors of the configuration shown at FIG. 3.

As shown at FIGS. 1 and 2, the preferred embodiment of the presentinvention comprises a cast housing 11 peripherally enclosed by a wall 12and having an internal wall 13 oriented generally transversely ofperipheral wall and intermediate the ends thereof. Peripheral wall 12and internall wall 13 are effective to form adjacent chambers 14 and 15each of which is open at the end opposite from the other chamber.Open-ended chambers 14 and 15 are normally closed by end closure plates16 and 17 respectively, which are secured to casting 11 by bolts 18threaded into tapped holes 19 in casting 11. End closure plates 16 and17 are also shown as being of cast construction and each is shown asbeing provided with fins 21 on the exterior thereof for the dissipationof heat. The exterior of wall 12 is also provided with fins 21 fordissipating heat.

Disposed within chamber 15 are a pair of lobed rotors 22 and 23 whichare identical, except for the possibility of normal minor variations inthe manufacture of otherwise identical elements. Rotors 22 and 23 areattached to spaced apart parallel shafts 24 and 25, respectively, bymeans of pins 26 and 27. Shaft 25 is journaled in spaced apart bearings28 and 29, located in chamber 14, and has a portion extending outwardlyfrom chamber 14 through closure plate 16 for connection to motive means(not shown). Shaft 24 is also journaled in spaced apart bearings locatedin chamber 14, viz bearings 31 and 32, and attached thereto is a gear 33disposed intermediate bearings 31 and 32. Gear 33 meshes with a similargear 34 attached to shaft 25 intermediate bearings 28 and 29. Thus,rotation of shaft 25 by means externally of the compressor will resultin rotation in opposite directions rotated in a clockwise direction,with resulting counterclockwise rotation of shaft 24 and rotor 22, wherewill be a compressing effect exerted by the rotors on fluid enteringchamber 15 through internally threaded connection 35 which will causethe fluid to exit from chamber 15 at higher pressure through outletconnection 36.

As shown at FIG. 3, the geometrical configuration of each of rotors 22and 23 comprises a plurality (shown as two) of lobes having theconfiguration of an epicycloid, and an equal number of recesses spacedbetween successive lobes and having the configuration of a hypocycloid.The dotted outline is the outline of a true alternatingepicycloid-hypocycloid rotor configuration which is generated about abase circle of diameter B by a point on the circumference of a rollingcircle of diameter A which alternately rolls for one revolution alongthe outside of the base circle and thence for one revolution along theinside of the base circle. In generating such a configuration thediameter of the rolling circle bears the following relationship to thediameter of the base circle: A=B/2N, where N is the number of desiredlobes in the resulting configuration. Thus, to generate the outline of adouble-lobed rotor, the relationship between the diameter of the rollingcircle and the diameter of the base circle is determined by the formulaA=B/4.

If a blower were constructed having a pair of identical rotors of theconfiguration of alternating epicycloids and hypocycloids according tothe formula A=B/2N and were disposed on parallel axes separated by adistance B, the diameter of the base circle, there would, in theory,

be line contact between the rotors throughout their rotation withneither clearance nor interference at any point of their rotation. Inpractice, of course, it is not desired to design for any contact betweenthe rotors because the rubbing action of rotor on rotor would generateexcessive heat at operating speeds of the order of up to 10,000 r.p.m.,with resulting excessive wear, and because inevitable manufacturingdeviations from the precise standard could lead to interference betweenthe rotors which would impair the operability of the blower.

Accordingly, it is necessary to depart from the rotor that required inview of the relative difiiculty involved in manufacturing to an optimumclearance, because excessive clearance results in increased slip andreduced volumetric efficiency. The points of necessary clearance betweenblower parts, listed in order of increasing importance from thestandpoint of difiiculty in manufacturing to an optimum clearance, areas follows:

(1) Between that point of each epicycloid lobe, which is radially mostremote from the axis of rotation of the rotor, and the inner surface ofwall 12 of the blower.

(In this regard it is noted that the configuration of the inner surfaceof wall 12 is that of the outline of intersecting circles of a diameter,D, which bears the relationship to the base circle: D=B+B/N, with eachof the intersecting circles drawn about centers at the axes of rotorshafts 24 and 25 which, in turn, are separated by a distance equal tothe diameter of base circle of each rotor.) This point of clearance isthe least critical because it is affected by the manufacturing precisionof only one rotor, the part whose dimensions are most difficult to hold,rather than both rotors.

(2) Between the two rotors when they are oriented at right angles to oneanother, as shown at FIG. 4: This point of clearance is more criticalbecause it is affected by the manufacturing precision of both rotors andby the axial spacing between the rotor shafts.

(3) Between the two rotors when they are oriented parallel to oneanother: In addition to the fact that this point of clearance isaffected by the manufacturing precision of both rotors and by thespacing of the rot-or shafts,

it is also affected by the angilar orientation of both rotors which, inturn, is affected by such factors as backlash in the driving gears andtorsional deflection in the rotor shafts.

Therefore, it is desirable to incorporate some modification to theconfiguration of a true rotor which will result in a satisfactoryclearance between the rotors when they are oriented parallel to oneanother; a lesser, but nonetheless still satisfactory clearance betweenthe rotors when they are oriented at right angles to one another; and aminimal, but still satisfactory clearance between the radially mostremote point of each lobe and the inner surface of the blower wall. Thisrelative priority of clearances can be met by constructing a rotor withalternating epicycloid lobes and hypocycloid recesses where theepicycloid lobe is generated by a point on the circumference of arolling circle whose diameter is less than that determined by theformula B/2N by a predetermined amount,

' C, and where the hypocycloid recesses are generated by a rollingcircle whose diameter is greater than that determined by the formulaB/2N by the same predetermined amount, C. This is shown at FIG. 3 wherethe partial outline of an actual rotor following the modifiedconfiguration is shown for the epicycloid lobe on the right and thehypocycloid lobe at the top.

As shown at FIGS. 4 and 5, a blower with rotors having the outline ofthe actual rotor of FIG. 3, will incorporate a design clearance of Cbetween the radially most remote point of the epicycloid lobe and theinternal surface of the surrounding blower wall. As shown at FIG. 4, Vthe rotors will have a design clearance equal to 2C when they aredisposed at right angles to one another. And as shown at FIG. 5, therotors will have a design clearance equal to 1rC when they are disposedparallel to one another. This increasing order of magnitude of designclearances matches nicely with the increasing order of difliculty inmanufacturing to a design clearance and is the only known rotor designformula which does so.

As an example, presented for the purpose of further illustrating anddisclosing the invention, it has been found that a blower withdouble-lobed rotors constructed in accordance with the presentinvention, wherein the diameter B of the base circle of each rotor wasone and onehalf inches, and wherein the difference C by which thediameter of the rolling circle for each epicycloid lobe was less thanthat determined by the formula was of the order of 0.001 inch, while thedifference by which the diameter of the rolling circle for eachhypocycloid recess exceeds that determined by the formula A=B/2N by thesame amount, was capable of operating within the specificationsestablished by a number of automobile manufacturers in regard toManAirOx exhaust purification systems.

In view of known manufacturing techniques for accurately geometricallydescribing a member whose extent can be expressed by an algebraicformula, there will be a number of manufacturing techniques obvious to askilled artisan for manufacturing rotors according to the foregoingconfiguration. As an example, it has been found that such rotors can bequite accurately manufactured in quantity lots from sintered metalpowders in accurately constructed dies. In the construction of the diethe outline of the rotor curve is developed in plywood at about timesactual size by means of a jig borer which operates accurately through acoordinate arrangement to both plot the points of the curve outline andto drill the holes. The plywood model is smoothed out and then reducedthrough one or more stages by means of a pantograph to make a veryaccurate template to be used by the die maker.

The best mode known to me to carry out this invention has been describedabove in terms .sufliciently full, clear, concise, and exact as toenable any person skilled in the art to make and use the same. It is tobe understood, however, that it is contemplated that other modes ofpracticing the invention can be made by a skilled artisan withoutdeparting from the scope of the invention which is defined only by theappended claims.

I claim:

1. In fluid compressor apparatus comprising, in combination: wall meansdefining a compressor chamber having an inlet for fluid to be compressedand an outlet for compressed fluid; a pair of rotatable lobed rotorsdisposed within the chamber on spaced axes; and means for rotating therotors in unison in opposite directions;

and characterized in that each rotor comprises a plurality of lobes ofsubstantially epicycloidal configuration and an equal number of recessesof substantially hypocycloidal configuration alternated betweensuccessive lobes, wherein each epicycloidal lobe is generated by a pointon the circumference of a rolling circle of a diameter not greater thanA rolling on a portion of the exterior of a base circle of a diameter Band wherein each hypocycloidal recess is generated by a rolling circleof a diameter not less than A rolling on an inside adjacent portion ofthe same base circle, in which A is determined by the formula A=B/2Nwhere N is equal to the number of lobes; and further characterized inthat the spacing between rotor axes is equal to B. I

2. Apparatus according to claim 1 wherein the wall means defining thecompressor chamber comprises a wall circumscribing the rotorscharacterized in that the inner surface of the wall generally followsthe outline of interthe formula D=B+B/N, and in that the intersectingcircles are generated about centers at the axes of the rotors.

3. Apparatus according to claim 1 wherein each epicycloidal lobe isgenerated by a rolling circle of a diameter AC having its dimensiondecreased by the value of the clearance factor C and each hypocycloidalrecess is generated by a rolling circle of a diameter A-l-C having itsdimension increased by the value of the clearance factor C, where C isnot materially in excess of 0.001 B.

4. Apparatus according to claim 3 wherein the wall means defining thecompressor chamber comprises a wall circumscribing the rotorscharacterized in that the inner surface of the wall generally followsthe outline of intersecting circles of a diameter D which is determinedby the formular D=B+B/N, and in that the intersecting circles aregenerated about centers at the axes of the rotors.

5. In fluid compressor apparatus comprising, in combination: wall meansdefining a compressor chamber having an inlet for fluid to be compressedand an outlet for compressed fluid; a pair of rotatable lobed rotorsdisposed within the chamber on spaced axes; and means for rotating therotors in unison in opposite directions; and characterized in that eachrotor comprises a pair of lobes of substantially epicycloidalconfiguration and a pair of recesses of substantially hypocycloidalconfiguration alternated between successive lobes, wherein eachepicycloidal lobe is generated by a point on the circumference of arolling circle of a diameter not greater than A rolling on a portion ofthe exterior of a base circle of a diameter B and wherein eachhypocycloidal recess is generated by a rolling circle of a diameter notless than A rolling on an inside adjacent portion of the same basecircle, in which A is determined by the formula A=B/2N where N is equalto the number of lobes; and further characterized in that the spacingbetween rotor axes is equal to B.

6. Apparatus according to claim 5 wherein the wall means defining thecompressor chamber comprises a wall circumscribing the rotorscharacterized in that the inner surface of the wall generally followsthe outline of intersecting circles of a diameter D which is determinedby the formula D=1.5B, and in that the intersecting circles aregenerated about centers at the axes of the rotors.

7. Apparatus according to claim 6 wherein each epicycloidal lobe isgenerated by a rolling circle of a diameter A-C having its dimensionincreased by the value of the clearance factor C and each hypocycloidalrecess is generated by a rolling circle of a diameter A+C having itsdimension increased by the value of the clearance factor C, where C isnot materially in excess of 0.001 B.

8. Apparatus according to claim 7 wherein the wall means defining thecompressor chamber comprises a wall circumscribing the rotorscharacterized in that the inner surface of the wall generally followsthe outline of intersecting circles of a diameter D which is determinedby the formula D=1.5B, and in that the intersecting circles aregenerated about centers at the axes of the rotors.

References Cited by the Examiner UNITED STATES PATENTS 166,295 8/1875Palmer et al. 230-141 2,547,392 3/1951 Hill et al. 103-126 2,666,3361/1954 Hill et al. 103-126 2,965,039 12/1960 Morita 103-126 2,988,0656/1961 Wankel et al. 103-126 2,994,277 8/1961 Merritt 103-126 3,089,6385/1963 Rose 230-141 3,105,634 10/1963 Hubrich 230-141 3,106,166 10/1963Tomasko et al 103-126 MARK NEWMAN, Primary Examiner.

WILBUR I. GOODLIN, Examiner,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION September 27, 1966Patent N00 3,275,225

Forrest O. E. Schultz It is hereby certified that error appears in theabove numbered patent requiring correction and that the said LettersPatent should read as corrected below.

Column 2, line 10, for "internall" read internal line 40, for "where"read there column 3, line 12, for "clearance" read clearances column 6,line 10, for "increased" read decreased --o Signed and sealed this 22ndday of August 19670 (SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner ofPatents

1. IN FLUID COMPRESSOR APPARATUS COMPRISING, IN COMBINATION: WALL MEANSDEFINING A COMPRESSOR CHAMBER HAVING AN INLET FOR FLUID TO BE COMPRESSEDAND AN OUTLET FOR COMPRESSED FLUID; A PAIR OF ROTATABLE LOAD ROTORSDISPOSED WITHIN THE CHAMBER ON SPACED AXES, AND MEANS FOR ROTATING THEROTORS IN UNISON IN OPPOSITE DIRECTIONS; AND CHARACTERIZED IN THAT EACHROTOR COMPRISES A PLURALITY OF LOBES OF SUBSTANTIALLY EPICYCLOIDALCONFIGURATION AND AN EQUAL NUMBER OF RECESSES OF SUBSTANTIALLYHYPOCYCLOIDAL CONFIGURATION ALTERNATED BETWEEN SUCCESSIVE LOBES, WHEREINEACH EPICYCLOIDAL LOBE IS GENERATED BY A POINT ON THE CIRCUMFERENCE OF AROLLING CIRCLE OF A DIAMETER NOT GREATER THAN A ROLLING ON A PORTION OFTHE EXTERIOR OF A BASE CIRCLE OF A DIAMETER B AND WHEREIN EACHHYPOCYCLOIDAL RECESS IS GENERATED BY A ROLLING CIRCLE OF A DIAMETER NOTLESS THAN A ROLLINGGON AN INSIDE ADJACENT PORTION OF THE SAME BASECIRCLE, IN WHICH A IS DETERMINED BY THE FORMULA A=B/2N WHERE N IS EQUALTO THE NUMBER OF LOBES; AND FURTHER CHARACTERIZED IN THAT THE SPACINGBETWEEN ROTOR AXES IS EQUAL TO B.